1. Field of the Invention
This invention relates to an apparatus and method for detecting vehicles or other magnetically permeable masses and measuring according to number, classification and speed and/or length.
2. Prior Art
Prior art traffic counters utilize road tube detection and magnetic loop sensing to detect the presence and/or movement of vehicles. The road tube counter comprises a flexible length of pressure tubing laid across the roadway. At one end of the tube, a pressure sensor is positioned to detect changes in the air pressure as wheels compress the tube. Disadvantages of road tubes are their susceptibility to damage and wear and their inability to count low speed vehicles. Magnetic loop sensors comprise a loop or coil of wire buried in a shallow trough in the roadway. The inductance of the coil due to the disturbance of the earth's magnetic field changes when a vehicle passes by. The change in inductance can be measured electronically. Disadvantages of the magnetic loop detector include installation requires tearing up the road, the detectors are susceptible to damage upon thermal expansion of the highway and they are unable to discriminate between closely passing vehicles.
Still another type of magnetic permeable sensor is described in U.S. Pat. No. 5,408,179 to Sampey et al. In this sensor, a ferromagnetic strip has a conductive winding wrapped about it. A small permanent magnet is positioned adjacent one end of the ferromagnetic strip. The magnet biases the ferromagnetic strip in a linear portion of its BH curve where the slope is substantially linear. An electronic circuit generates an analog signal indicative of the inductance of the winding as the earth's magnetic field is disturbed. Another electronic circuit digitizes the analog signal at spaced time intervals to produce a series of digitized values. A microprocessor processes the digitized values to produce a first time series profile that characterizes the presence and/or motion of the magnetic permeable mass. Another sensor, similar to the above-described sensor, is spaced apart from the above-described sensor a fixed distance in the direction of the travel of the magnetically permeable mass. The output of the second sensor is also digitized by the electronic circuit to produce another series of digitized values. The microprocessor processes these digitized values to produce a second time series profile and determines equivalent positions in the first time series profile and second time series profile.
A disadvantage of this sensor is that a 100 Khz excitation signal utilized to stimulate the conductive winding also produces undesirable noise in other components of the electronic circuitry.
It is, therefore, an object of the present invention to provide a new apparatus and method for detecting characteristics of a magnetically permeable mass and detecting a speed of a magnetically permeable mass that avoids the above disadvantages and others. Still other objects of the invention will become apparent to one of ordinary skill in the art upon reading and understanding the ensuing detailed description.